The present invention relates generally to an optical correlator, and in particular to an asymmetric single shot autocorrelator.
Historically, early optical correlators measured the intensity correlation function of two light pulses by combining them together in a nonlinear optical media to generate a sum frequency signal as described in E. P. Ippen and C.V. Shank, xe2x80x9cTechniques for measurementxe2x80x9d in Ultrashort Light Pulses-Picosecond Techniques and Applications, S. L. Shapiro, ed., 83 (Springer-Verlag, Berlin, 1977), herein incorporated by reference in its entirety. The intensity of the sum frequency signal was then measured as a function of relative pulse delay. In the case where the pulse is combined with its own replica, the result of such a measurement is an autocorrelation function of the pulse. Since the optical frequency of the nonlinear output in this case equals the frequency of the second harmonic of the pulse, nonlinear mechanisms employed in these autocorrelators are referred to as second harmonic generation (xe2x80x9cSHGxe2x80x9d). Adjustable time delay between the two pulses can be introduced either by using a regular scanning optical delay line or by overlapping wide and spatially uniform pulsed beams intersecting at an angle. In the latter case, the measurement of the correlation function can be performed with only one light pulse as discussed in R. N Gyuzalian., S. B. Sogomonian and Z. G. Horvath, xe2x80x9cBackground-free measurement of time behavior of an individual picosecond laser pulsexe2x80x9d, Opt. Commun. 29, 239-242 (1979), (xe2x80x9cthe Gyuzalian referencexe2x80x9d), herein incorporated by reference in its entirety. This experimental arrangement is known as a xe2x80x98single shot autocorrelationxe2x80x99.
Optical correlators, while simple, have an important drawback. Optical correlators can not derive an actual time dependent optical E-field of a pulse based on the measured correlation function. To overcome this disadvantage, several new techniques were developed as discussed in J. L. A. Chilla and O. E. Martinez, xe2x80x9cDirect measurement of the amplitude and the phase of femtosecond light pulsesxe2x80x9d, Opt. Lett. 16, p. 39-41 (1991), D. J. Kane and R. Trebino, Opt. Lett. 18, p. 825 (1993), IEEE Journal of Quantum Electron. 29, p. 571-579, (1993). (xe2x80x9cthe Kane referencexe2x80x9d), C. Iaconis and I. A. Walmsley xe2x80x9cSpectral phase interferometry for direct electric-field reconstruction of ultrashort optical pulsesxe2x80x9d Opt. Lett., 23, pp. 792 (1998) (xe2x80x9cthe Iaconis referencexe2x80x9d), M. Beck, M. G. Raymer, I. A. Walmsley and V. Wong xe2x80x9cChronocyclic tomography for measuring the amplitude and phase structure of optical pulsesxe2x80x9d, Opt. Lett. 18, pp. 2041 (1993), Y. T. Mazurenko, S. E. Putilin, A. G. Spiro, A. G. Beliaev, V. E. Yashin and S. A. Chizhov xe2x80x9cUltrafast time-to-space conversion of phase by the method of spectral nonlinear opticsxe2x80x9d Opt. Lett. 21, pp. 1753 (1996), V. Kabelka, A. V. Masalov xe2x80x9cTime-frequency imaging of a single ultrashort light pulse from angularly resolved autocorrelationxe2x80x9d, Opt. Lett. 20, pp. 1301 (1995), and D. J. Kane and R. Trebino, Opt.Lett. 18, pp. 823 (1993) (the Trebino referencexe2x80x9d), herein incorporated by reference in their entirety.
Presently, the most commonly used techniques for ultrashort light pulse characterization are FROG (Frequency Resolved Optical Gating) as in the Kane reference and SPIDER (Spectral Phase Interferometry for Direct Electric-field Reconstruction) as in the Iaconis reference. Both techniques are available commercially and were successfully used to measure properties of light pulses shorter than 10 femtoseconds as discussed in G. Taft, A. Randquist, M. Murnane, I. P. Christov, H. Kapteyn, K. DeLong, D. Fittinghoff, M. Krumbugel, J. Sweetser and R. Trebino xe2x80x9cMeasurement of 10-fs Laser pulsesxe2x80x9d IEEE J. Sel. Top. Quantum Electron. 2, pp. 575-585 (1996) and L. Gallmann, D. H. Sutter, N. Matushek, G. Steinmeyer, U. Keller, C. Iaconis and I. A. Walmsley xe2x80x9cCharacterization of sub-6-fs optical pulses with spectral phase Interferometry for direct electric-field reconstructionxe2x80x9d, Opt. Lett. 24, pp. 1314 (1999), herein incorporated by reference in their entirety.
SPIDER is an interferometric technique where the optical phase of the pulse is determined from a one-dimensional pattern, formed in frequency-domain as a result of interference between two time-separated replicas of the pulse whose spectra are frequency-shifted. The output of the SPIDER is the intensity of this interferometric pattern measured as a function of optical frequency. A very important advantage of SPIDER is that this method does not require an iterative algorithm to retrieve the pulse E-field. In addition, SPIDER allows the measurement not only of magnitude of pulse chirp, but its overall sign as well. However, SPIDER has a more complicated optical scheme: it requires three replicas of input pulse to form the interference pattern (instead of two replicas, as in case of SHG-FROG).
SHG-FROG is essentially a spectrally resolved optical correlator, where an optical spectrum of the nonlinear signal from the correlator is recorded as a function of optical delay. The result of an SHG-FROG measurement can be presented as a two-dimensional image, where intensity is a function of delay time and optical frequency. This image is also known as an SHG-FROG pattern. Data contained in such a pattern are sufficient to compute the E-field of the pulse resulting in the pattern.
Compared to SPIDER, SHG-FROG is easier to implement experimentally. It also appears to be less sensitive to experimental noise and calibration errors since the two-dimensional SHG-FROG pattern is actually mathematically redundant for pulse retrieval purposes. However, a range of possible applications for SHG-FROG is limited because SHG-FROG requires numerically intensive data processing procedures to retrieve the optical phase and intensity of a pulse from the measured SHG-FROG pattern as discussed in D. J. Kane and R. Trebino, Opt. Lett. 18, pp. 823 (1993) (xe2x80x9cthe Trebino referencexe2x80x9d) and K. W. DeLong, D. Fittinghoff, R. Trebino, B. Kohler and K. Wilson xe2x80x9cPulse retrieval in frequency-resolved optical gating based on the method of generalized projectionsxe2x80x9d, Opt. Lett. 19, pp. 2152-2154 (1994) (xe2x80x9cthe DeLong referencexe2x80x9d) and xe2x80x9cSimultaneous measurement of two ultrashort laser pulses from a single spectrogram in a single shot,xe2x80x9d D. J. Kane, G. Rodriguez, A. J. Taylor, and T. S. Element, JOSA B 14, pp. 935-943 (1997), herein incorporated by reference in their entirety. Another disadvantage specific to both an SHG autocorrelator and an SHG-FROG is the inability of these diagnostics to establish the direction of time axis, so that the pulse and time-reversed replica are distinguishable. All these disadvantages limit the possibility to use SHG-FROG as a diagnostic tool for real time low-intensity applications.
It is thereby desirable to design an ultrashort light pulse characterization system that solves the aforementioned problems.
The present invention is directed to a method for characterization of a light pulse such that time ambiguity of the pulse is removed and the sign of the phase modulation of at least one input beam is determined. First, a first fundamental beam and a second fundamental beam are optically delayed in a nonlinear crystal. The optical delay of the first fundamental beam and the second fundamental beam is:
xcfx84=2 sin(xcex1/2)x/c; 
wherein xcex1 represents the intersecting angle of the first fundamental beam and the second fundamental beam; x represents a vertical coordinate position at a surface of the crystal; c represents the speed of light in a vacuum. Then, nonlinearity is introduced to the first fundamental beam and the second fundamental beam through a nonlinear mechanism. Nonlinear mechanisms can include, but are not limited to sum frequency generation, polarization gating, and self-diffraction. Finally, asymmetry is imputed to at least one of the nonlinear beams by using a blocking mask. In particular, the following asymetric (-auto) correlation function can be obtained:
IAAC(xcfx84)xe2x88x9d∫|2E(t+xcfx84)E(t)+E2 (t)|2dt 
wherein E(t) represents a time domain analytic signal of the light beam; and xcfx84 represents optical delay between the first fundamental beam and the second fundamental beam.